P 1039 – Enhanced Continuum Damage Mechanics Model for Low Triaxialities for the Deep Drawing Simulation of Advanced High Strength Steels
Advanced high strength steels are still a common choice for lightweight design in the automotive industry. Due to their good performances such as high strength and high energy absorption, these steel grades are excellent for body in white components. Because of their restricted ductility, which sometimes leads to the formation of cracks without or with low necking during forming operations, the conventional forming limit diagrams may fall short. As a remedy, continuum damage models are commonly applied in the literature to predict failure due to fracture.
In this work, a variant of Lemaitre’s continuum damage model (CDM) is investigated to predict failure during deep drawing of DP1000 steels. Previous investigations (in project P853) showed that the model extension (quasi-unilateral damage evolution), which scales the damage evolution due to compressive stress states, improves the failure predictions. Especially for the forming processes in which the fracture occurs dominantly under shear, the onset and location of the fracture can be predicted successfully. However, the fracture locus predicted by this model for low and negative stress
triaxialities is limited. Reduced ductility under shear and plane-strain cases, on the contrary to the high fracture strains under biaxial forming conditions cannot be provided at the same time. To solve this drawback, Lemaitre’s CDM has been enhanced by considering the maximal shear stress, such that the model capability is extended to obtain predictions close to the experimental observations. To predict the onset of the strain localization, two methodologies for the prediction of instability are combined with this model (for the commercial FE-programs LS-DYNA and ibura).
The inverse parameter identification for the material characterization is designed such that the predictions cover a large range of the stress states occurring during sheet forming operations. Tensile tests with notched and holed specimens and Nakajima-test with biaxial specimens are used for the characterization of the forming behavior between uniaxial tension and biaxial tension. For the material characterization under shear, several shear tests are compared with other and an appropriate testing method for thin sheets (1 mm) using in-plane torsion tests is introduced. Optimization strategies with different test combinations are discussed to reduce the computational effort for the parameter identification. The validation studies include deep drawing of several parts, such as a square cup, a cross-die specimen and a component relevant for the automotive industry. The predictive performance of the presented model is evaluated by the comparisons between experiments and simulations of those validation tests.
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Prof. Dr.-Ing. A. E. Tekkaya, Dr.-Ing. T. Clausmeyer, K. Isik M.Sc., Dr.-Ing. M. Goretti Doig